This invention relates generally to the field of enzymatic conversion of lignocellulosic material to monomeric sugars and particularly to maximizing enzyme performance during a liquefaction stage of pretreated lignocellulosic material.
Enzymatic hydrolysis of pretreated lignocellulosic material poses many challenges. These challenges go from the interaction of the enzymes themselves with the biochemical complexity of the pretreated material and its derivatives and to the physical characteristics of the liquid/fiber, monomeric/oligomeric mixture (collectively referred to as a “slurry”) and its rheological features.
A conventional reactor to accomplish enzymatic hydrolysis required large batch tanks having expensive and powerful impellers to mix the enzymes into the slurry. Enzymatic liquefaction of lignocellulosic material may require several hours of mixing in the large batch tanks. The mixing process reduces the viscosity of the lignocellulosic material by converting the material from a generally solids composition to a liquefied slurry. The pretreated lignocellulosic material typically starts the mixing and process having a mud-like consistency.
Lignocellulosic material is pretreated and subjected to enzymatic conversion to monomeric sugars. The enzymes added to the lignocellulosic material typically have a relatively low concentration with respect to the lignocellulosic material. The lignocellulosic material and enzyme mixture tends to be highly viscous as it enters a mixing and pretreatment reactor system. The high viscosity of the mixture has motivated the use of relatively small reactor vessels to reduce the torque needed to mix the mixture. Such a system typically includes one or more hydrolysis reactor vessels.
The mixing vessels of a conventional mixing and pretreatment reactor system for enzymatic liquefaction of lignocellulosic material have traditionally been operated in a batch mode rather than a continuous mode. A batch mode for mixing is often better suited to situations were several smaller mixing vessels feed a larger downstream vessel, such as a digester or other reactor vessel.
Recirculation of liquefied material to dilute the incoming pretreated lignocellulosic material has been proposed to decrease the viscosity, and improve the mixing. Recirculation has a disadvantage in that additional mixing volume is required to achieve the desired retention time in the vessel. Batch processing adds volume to the system, as time has to be provided to fill and empty the vessel.